This invention relates to a scale factor changing mechanism in a copying machine.
In order to change a scale factor in a copying machine, it is necessary to change the distance between the original's surface and a lens and the distance between the lens and a photosensitive surface. However, since the positions of the original and the photosensitive member are fixed because of the structure of the copying machine, the length of the optical path must therefore be corrected by displacing the lens and simultaneously by changing the positions of mirrors.
An electrographic copying machine as shown in FIG. 1 is well known in the art in which four mirrors are used in order to make the optical system compact. In this copying machine, the image of an original S on a platen of glass P is formed on the surface of a photosensitive drum D by means of a first mirror M.sub.1, a second mirror M.sub.2, a lens L, a third mirror M.sub.3 and a fourth mirror M.sub.4. In the case where two mirrors are interposed between the surface of the original and the lens and two mirrors are set between the lens and the photosensitive surface as described above, displacement of the mirrors for varying a scale factor is not simple parallel movement. Accordingly, the mirror position adjusting mechanism is necessarily intricate. Furthermore, in the above-described arrangement, the distance between the platen P and the photosensitive drum D is relatively large. This makes it difficult to sufficiently miniaturize the copying machine. Thus, attempts have been made to define systems where the mirrors are movable.
In a known mirror movement type electrographic copying machine in which four mirrors are arranged as shown in FIG. 2, first, second and third mirrors M.sub.1, M.sub.2 and M.sub.3 are disposed between an original S and a lens L. A fourth mirror M.sub.4 is set between the lens L and a photosensitive drum D. In this copying machine, the distance between the platen P and the photosensitive drum D is relatively small, which makes it possible to miniaturize the copying machine. The original is scanned by moving the first mirror M.sub.1 in parallel with the surface of the original and moving the second and third mirrors M.sub.2 and M.sub.3 (which are supported by the same frame, not shown) in the same direction at a speed which is a half of the speed of the first mirror M.sub.1. A scale factor can be changed by moving the lens L and the second and third mirrors M.sub.2 and M.sub.3 in parallel with the direction of the arrow A (or the direction of the optical axis of the lens). Accordingly, the mirror position adjusting mechanism for the copying machine in FIG. 2 is simpler than that for the copying machine in FIG. 1. However, the mechanism suffers from drawbacks. For example, since the mirrors M2 and M3 are held on the same frame and are heavy, the device has significant inertia. Shocks are caused when the mirrors are started or stopped for position adjustment and large power is required for position adjustment.
In order to overcome these disadvantages, a method in which instead of moving the second and third mirrors M.sub.2 and M.sub.3, the fourth mirror M.sub.4 is moved, may be considered. In the case where the method is employed, it is necessary that, as shown in FIG. 3, while the fourth mirror M.sub.4 is being moved in the direction of the arror A, its angle of inclination (.alpha.) with the optical axis of the lens must be changed in order that a projection point on the photosensitive drum D is maintained unchanged. Accordingly, the mirror position adjusting mechanism is intricate. Furthermore, when a scale factor is changed, the exposure incident angle with respect to the surface of the photosensitive drum D is changed, and therefore the quality of picture may be lowered depending on a selected scale factor.